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E-cadherin/catenin complex in glomerulonephritis
Lydia Nakopoulou, MD and Aphrodite Nonni, MD
Department of Pathology, Medical School,
The National and Kapodistrian University of Athens,
Cadherin/catenin complexes have a crucial role in cell-cell and cell-matrix adhesion; for maintenance of normal intercellular adhesion, an intact E-cadherin/catenin complex is required.
Cadherins are a large family of transmembrane glycoproteins, that mediate calcium-dependent cell-cell adhesion in normal epithelial cells and play essential role in development, establishment and maintenance of cell polarity and tissue architecture. E(pithelial)-cadherin is the prototypic member of the type-I cadherin superfamily. It has an extracellular domain consisting of five cadherin-type repeats, that interact in a calcium-dependent fashion with analogous molecules on adjacent cells, forming a "molecular zipper". The extracellular part of E-cadherin interacts mainly homotypically (epithelial cell to epithelial cell), but also heterotypically (melanocytes or dendritic cells to keratinocytes). In addition, it interacts with another molecules, such as αEβ7 integrin or internalin. The cytoplasmic domain of E-cadherin binds directly to either β-catenin or γ-catenin (also called plakoglobin). This complex is coupled to the cytoskeleton by α-catenin via its association with actin and β-catenin or γ-catenin.
The E-cadherin/catenin complex is implicated not only in cell-cell adhesion, but in directed migration during epithelial wound healing, presumably through contact inhibition of membrane ruffling. Also, it is a pivotal molecule in signal transduction from the outer cell surface to the cytoskeleton. The E-cadherin/catenin complex is a substrate of receptor tyrosine kinases e.g. c-erbB-2, c-met, EGFR, of non-receptor tyrosine kinases e.g. src, of receptor protein tyrosine phosphatases and of MUC-1 (episialin).
Disruption of the complex or its disconnection with other signal-transducing systems may be involved in any stage of carcinogenesis, tumour invasion and metastasis. Mutations of the E-cadherin gene (CDH1), which is located on chromosome 16q22.1 and acts as a tumour-suppressor gene, have been described in several epithelial cancers, especially in lobular breast and diffuse gastric carcinomas.
Catenins have been identified as cytoplasmic anchorage proteins for cadherins. β-Catenin protein is a member of the "armadillo" superfamily and was originally described as an element of the E-cadherin/catenin complex. It was initially considered to be a cell-cell adhesion protein. Nowadays, it has been established, that this protein is also behaved as a signal transduction molecule in developmental systems, being an element of the Wnt (wingless/int) signalling pathway. In normal cells, β-catenin is associated not only with cadherins but also with the APC (adenomatous polyposis coli) multi-protein complex. β-Catenin may be involved in tumourogenesis. α-Catenin functions through maintenance of cell-cell adhesion and may interference with β-catenin/Tcf/DNA complex formation and β-catenin signalling in the nucleus. γ-Catenin (plakoglobin) is a close homologue of β-catenin. It binds independently to E-cadherin in the adherens junctions and also to the desmosomal cadherins desmoglein and desmocollin. P120 catenin (p120CTN) is a member of a large subfamily of Armadillo proteins. It can be found at the membrane and in the nucleus like β-catenin. It is hypothesized, that p120CTN existing in an "active" form, induces cadherin clustering and cell-cell adhesion by binding to the juxtamembrane part of E-cadherin, and that intracellular signalling can induce an "inactive" form of p120CTN. Catenins, namely β-catenin, plakoglobin and p120CTN are good substrates for tyrosine kinases.
Many studies have correlated changes in cadherin expression with the onset of such morphoregulatory processes at tissue rearrangement, cell migration, proliferation, apoptosis, and differentiation; misregulated cadherin expression can alter characteristics of differentiated cells. Most of the processes potentially are involved in the pathogenesis of glomerulonephritis (GN). To date, only adhesion molecules with direct participation in the inflammatory response (such as integrins) have been studied in GNs. The expression of such cell-cell adhesion molecules as E-cadherin and E-cadherin complex-associated catenins has not been investigated to date.
In our previous study, the expression of these adhesion molecules has been evaluated, in a well-documented series of 95 kidney biopsies from 51 patients with various types of primary GN and 49 patients with already diagnosed systemic lupus erythematous (SLE) and GN. The cases were studied immunohistochemically, using specific monoclonal antibodies against E-cadherin, α-catenin, β-catenin, γ-catenin and p120CTN.
From our results, in normal controls, all molecules were detectable with a membrane-staining pattern exclusively in distal and collecting tubules, whereas glomeruli were constantly negative. The same distribution was observed in cases with GNs, but the intensity was stronger. Among the examined markers, p120CTN, along with β- and γ-catenins, were generally detected more often than the rest and their expression was focused mainly in parietal epithelium. Distal and collecting tubules, almost constantly, were immunopositive for E-cadherin, while this molecule was barely detectable in glomerular cells and practically absent in cellular crescents or microadhesions. The latter structures were frequently positive for β- and γ-catenins and p120CTN. Mesangial cells were positive for p120CTN and β-catenin in severely hyperplastic areas. Fibrotic glomeruli always were negative for all markers. Comparing primary and secondary GNs, the latter (lupus-associated GNs) showed more frequent immunopositivity for p120CTN and γ-catenin. With the statistical analysis, β-catenin in parietal epithelial cells, as well as in proximal and collecting tubules, was inversely related to the respective expression of p120CTN.
Adhesion molecules, as well as growth factors, are normally expressed during nephrogenesis. The former includes molecules that mediate the attachment of cells to one another, such as E-cadherin. In developing kidney, E-cadherin is expressed in the upper limb of S-shaped bodies, structures that will develop into distal and collecting tubules; the latter were found to express E-cadherin in the vast majority of the specimens. Other members of the cadherin family have been expressed specifically in proximal renal tubules of normal kidneys. With regard to β-catenin, its protooncogene is a candidate target molecule of Wnt-4 signaling; Wnt-4 acts as an autoinducer of the mesenchymal to epithelial transition that underlies normal nephron development. This transition probably is associated with alterations in adhesion molecule expression, and this modified adhesion permits morphogenic movement during tissue remodeling. Thus, we see that cell-cell adhesion, which appears as a tight and stable mechanism preserving tissue integrity, can be modified under certain circumstances. This concept made us examine the expression of E-cadherin and its related catenins in glomerular epithelium and lesions deriving from it.
According to our results, β- and γ-catenins, as well as p120CTN, were expressed on glomerular epithelium in patients with GN, as well as in lesions deriving from it (ie, cellular crescents and epithelial cells around microadhesions). Crescents contain a mixture of cells, principally glomerular epithelium and macrophages. The proliferating epithelium in crescents is predominantly parietal in origin; visceral epithelium rarely proliferates. In the samples examined, expression of the three previously mentioned molecules was more prominent in parietal epithelium than podocytes. It is known, that proliferating epithelial cells within crescents form a relatively cohesive mass by interactions between unregulated surface membrane adhesion molecules that, according to our results, include β- and γ-catenins, as well as p120CTN. Stimulation of epithelial proliferation is based on growth factors; epithelial cells in culture (parietal in origin in most studies) proliferate in response to EGF. Exposure to a number of growth factors, including EGF, transforming growth factor-β, platelet-derived growth factor, and hepatocyte growth factor, results in direct or indirect phosphorylation of both catenin and p120CTN family members. β-Catenin, in particular, has been reported to form complexes with the EGF receptor. β-Catenin and γ-catenin, as well as p120CTN, are targets of tyrosine phosphorylation, which causes them to dissociate from the cadherin-associated complex and thus disrupt adhesion. This is probably related to be restricted E-cadherin expression of our specimens. In many cases, the adhesive response of the cell to particular mitogens shows much about how the extracellular environment affects the establishment or maintenance of intercellular adhesion. Therefore, the selective epithelial expression of β-catenin, γ-catenin and p120CTN in cellular crescents and microadhesions, likely is linked with the epithelial cell’s response to various mitogens. The association of these three molecules with epithelial expression and lupus-associated proliferative GN, probably exists because crescents in immune complex-mediated GN, appear to be composed predominantly of unregulated epithelial cells, which can express these adhesion molecules.
In addition to the mitogenic role of growth factors on epithelial glomerular cells, other mediators that belong to the category of proinflammatory cytokines and can be produced by mesangial cells, eg, monocyte chemoattractant protein-1, are involved in the activation and persistent glomerular infiltration of inflammatory cells by upregulating the expression of intercellular adhesion molecule-1 (ICAM-1). In contrast to ICAM-1, β-, γ-and p120CTN catenins as well as their potential upregulators , do not appear to participate in crescent formation by affecting the glomerular accumulation of monocytes / macrophages; however, they probably do so by upregulating glomerular epithelial cells.
According to all above, p120CTN and γ-catenin are expressed frequently in epithelial cells of diseased glomeruli and, in contact with β-catenin, appear to participate in the pathogenesis of cellular crescents or microadhesions, especially in lupus-associated GN. However, further investigation using molecular methods in tissue specimens is needed to confirm all our suggestions.